Ruthenium Complexes of 2-[(4-(Arylamino)phenyl)azo]pyridine Formed via Regioselective Phenyl Ring Amination of Coordinated 2-(Phenylazo)pyridine:  Isolation of Products, X-ray Structure, and Redox and Optical Properties

Aromatic ring amination reactions in the ruthenium complex of 2-(phenylazo)pyridine is described. The substitutionally inert cationic brown complex [Ru(pap)₃](ClO₄)₂ (1) (pap = 2-(phenylazo)pyridine) reacts smoothly with aromatic amines neat and in the presence of air to produce cationic and intense blue complexes [Ru(HL²)₃](ClO₄)₂ (2) (HL² = 2-[(4-(arylamino)phenyl)azo]pyridine). These were purified on a preparative TLC plate. The X-ray structure of the new and representative complex 2c has been solved to characterize them. The results are compared with those of the starting complex, [Ru(pap)₃](ClO₄)₂ (1). The transformation 1 → 2 involves aromatic ring amination at the para carbon (with respect to the diazo function) of the pendant phenyl rings of all three coordinated pap ligands in 1. The transformation is stereoretentive, and the amination reaction is regioselective. The extended ligand HL² coordinates as a bidentate ligand and chelates to ruthenium(II) through the pyridine and one of the azo nitrogens. The amine nitrogen of this bears a hydrogen atom and remains uncoordinated. Similarly, the amination reaction on the mixed-ligand complex [Ru(pap)(bpy)₂](ClO₄)₂ produces the blue complex [Ru(HL²)(bpy)₂](ClO₄)₂ (3) as anticipated. The reactions of [RuCl₂(dmso)₄] and [Ru(S)₂(L)₂]²⁺ (dmso = dimethyl sulfoxide, S = labile coordinated solvent, L = 2,2‘-bipyridine (bpy) and pap) with the preformed HL² ligand have been explored. The structure of the representative complex [RuCl₂(HL^2a)₂] (5a) is reported. It has the chlorides in trans configuration while the pyridine as well as azo nitrogens are in cis geometry. Optical spectra and redox properties of the newly synthesized complexes are reported. All the ruthenium complexes of HL² are characterized by their intense blue solution colors. The lowest energy transitions in these complexes appear near 600 nm, which have been attributed to intraligand charge-transfer transitions. For example, the lowest energy visible range transition in [Ru(HL^2b)₃]²⁺ appears at 602 nm and its intensity is 65 510 M^-1 cm^-1. All the tris chelates show multiple-step electron-transfer processes. In [Ru(HL²)₃]²⁺, six reductions waves constitute the complete electron-transfer series. The electrons are believed to be added successively to the three azo functions. In the mixed-ligand chelates [Ru(HL²)(pap)₂]²⁺ and [Ru(HL²)(bpy)₂]²⁺ the reductions due to HL², pap, and bpy are observed.